Ventilator system and method

ABSTRACT

The ventilator system and method use a isolating heat exchanger to selectively transfer heat between exhaust air leaving an enclosed space and outside air entering the enclosed space. The system operates in three basic modes, under the control of a microprocessor-based controller which is responsive to the temperatures inside and outside of the enclosed space. In the heating mode, heat is transferred from the exhaust air to the outside air when the enclosed space requires heating. In the cooling mode, heat is transferred from the outside air to the exhaust air when the outside air temperature is higher than that in the enclosed space. In the supplemental cooling mode, heat transfer between the exhaust and outside air is reduced or eliminated when the outside air temperature is below the desired temperature in the enclosed space and cooling is required. In the heating mode, the heat exchanger is selectively heated when necessary to defrost it. A by-pass is provided for the introduction of outside air in order to avoid heating of the outside air by the exhaust air during the supplemental cooling mode of operation, and/or provide fresh air to the enclosed space without cooling the heat exchanger while it is being defrosted.

[0001] This invention relates to ventilator systems and methods, andparticularly to ventilator systems and methods using isolating heatexchangers to exchange heat between exhaust air leaving an enclosedspace, and outside air entering the building.

[0002] It is increasingly common to make modern homes and commercialbuildings very air-tight in order to reduce heating and cooling costs.As a result, it has become increasingly necessary to bring fresh airinto the building in order to avoid an extreme reduction of oxygen inthe building, which can cause the illness or even death of theoccupants.

[0003] Prior ventilator systems have been designed to use heatexchangers to transfer heat from the exhaust air to the outside air topre-heat the outside air before it enters the building, thus savingenergy and heating costs. This is useful in cold weather when thebuilding must be heated.

[0004] In order to protect the health of the occupants, it is verydesirable to prevent the air being exhausted from the enclosed spacefrom mingling with the incoming outside air. Thus, often the heatexchangers which are used are those in which the flow of exhaust air isisolated from the flow of outside air through the heat exchanger. Theseare sometimes called “isolating” heat exchangers.

[0005] Other ventilator systems provide for heat transfer from theoutside air to the exhaust air when the outside air temperatures arehigher than the desired temperatures in the enclosed space. This coolsthe incoming air before it is further refrigerated to provide theultimate desired temperature, thus reducing the load on therefrigeration system and reducing the cost of cooling.

[0006] Applicant has discovered a problem in that it is believed thatprior ventilator systems are not efficiently operable throughout acomplete range of outside and inside air temperature conditions. Thisreduces the utility of the prior ventilator systems and makes them lesscost-effective than they might be.

[0007] Accordingly, it is an object of the present invention to providea ventilating system and method in which the foregoing problems areeliminated or alleviated.

[0008] More particularly, it is an object of the invention to provide aventilator system and method which can be utilized for essentially allventilating conditions, and at essentially all times of the year.

[0009] It is an object of the present invention to provide a flexibleventilator system and method in which the mode of operation can bechanged readily in response to changing climactic and interior spaceconditions.

[0010] It is a further object of the invention to provide such a systemwhich is relatively inexpensive to manufacture, simple in construction,inexpensive to install and use, and reliable for long- term operation.

[0011] In accordance with the present invention, a ventilating systemand method are provided in which an isolating heat exchanger is used forrecovering energy while supplying outside air to an enclosed spaceduring all weather conditions, by the use of inside and outside airtemperature signal.

[0012] The invention also provides a ventilating system and method usingan isolating heat exchanger in which two different modes of cooling areavailable; one in which the heat exchanger transfers heat from theincoming outside air to the cooler exhaust air, and another in whichheat transfer between the incoming and outgoing air is temporarilyeliminated in order to provide supplemental cooling when the inside airtemperature is below the inside air temperature.

[0013] Additional cooling is provided by evaporatively cooling theexhaust air before it reaches the heat exchanger so as to greatlyincrease the temperature drop of the incoming outside air before itenters the enclosed space.

[0014] A further feature of the invention is the provision of aventilating system and method using an isolating heat exchanger withsupplemental cooling, without heat transfer between incoming andoutgoing air, as well as heat transfer from the exhaust air to theincoming outside air during cold weather.

[0015] Another feature of the invention is the provision of a mode ofoperation to defrost the heat exchanger. This operation is accomplishedby any one or more of the methods of reducing outside air flow;by-passing outside air flow around the heat exchanger; and heating theoutside air before it reaches the heat exchanger.

[0016] The preferred ventilator system and method use a isolating heatexchanger to selectively transfer heat between exhaust air leaving anenclosed space and outside air entering the enclosed space. The systemoperates in three basic modes, under the control of amicroprocessor-based controller which is responsive to the temperaturesinside and outside of the enclosed space. In the heating mode, heat istransferred from the exhaust air to the outside air when the enclosedspace requires heating. In the cooling mode, heat is transferred fromthe outside air to the exhaust air when the outside air temperature ishigher than that in the enclosed space. In the supplemental coolingmode, heat transfer between the exhaust and outside air is reduced oreliminated when the outside air temperature is below the desiredtemperature in the enclosed space and cooling is required. In theheating mode, the heat exchanger is selectively heated when necessary todefrost it. A by-pass is provided for the introduction of outside air inorder to avoid heating of the outside air by the exhaust air during thesupplemental cooling mode, and/or provide fresh air to the enclosedspace without cooling the heat exchanger while it is being defrosted.

[0017] The foregoing and other objects and advantages of the inventionwill be set forth in or apparent from the following description anddrawings.

IN THE DRAWINGS

[0018]FIG. 1 is a schematic side-elevation and cross-sectional view of aventilating system constructed in accordance with the present invention;

[0019]FIG. 2 is a perspective view of a heat exchanger used in thesystem shown in FIG. 1;

[0020]FIG. 3 is a perspective, broken away enlarged and partiallyschematic view of a portion of the heat exchanger shown in FIG. 2;

[0021]FIG. 4 is a schematic diagram of a control circuit for controllingthe operation of the system shown in FIG. 1; and

[0022]FIG. 5 is a perspective, partially exploded view of a roof-topinstallation of the ventilator of the present invention.

GENERAL DESCRIPTION

[0023]FIG. 1 shows the ventilating system 10 of the present invention inschematic form.

[0024] The ventilating system 10 is in use to provide ventilation to anenclosed space indicated schematically by the dashed line 12. Thisenclosed space can be a residence, a business office, a skyscraper orother type of enclosed space.

[0025] The ventilating system 10 includes a housing 14 in which ismounted a heat exchanger 16, an exhaust air fan 28, and an outside airfan 26.

[0026] The heat exchanger 16 is mounted in the housing 14, tilted asshown in FIG. 1, with its upper and lower edges 42 and 44 sealed to thetop and bottom walls of the housing, respectively, and with the othercorners of the heat exchanger attached with sealing members 38 and 40 tothe side walls of the housing, also so as to provide an air seal betweenadjacent sides of the heat exchanger.

[0027] The unit described so far is supplied as a free-standing module.Attached to it, as in a typical installation, is an outside air inletduct 18 and an exhaust air outlet duct 20. Both of the ducts 18 and 20communicate with the ambient air outside of the enclosed space 12.

[0028] Attached at the other end of the housing 14 are an exhaust inletduct 22 and an outside air outlet duct 24. The mounting of the heatexchanger 16 in the housing, with the seals at its four corners,provides two isolated air flow paths through the heat exchanger.

[0029] One path is shown by arrows 46 and 48 extending from the outsideair inlet duct 18 downwardly and to the right and out through the duct24, in the direction shown by the solid arrows through the heatexchanger 16.

[0030] The other flow path is from the exhaust inlet duct 22 through theheat exchanger 16 in the direction of the arrow 52 and out through theexhaust outlet duct 20, as shown by the dotted arrows passing throughthe heat exchanger in FIG. 1.

[0031] The flow paths taken by the outside air and the exhaust airthrough the heat exchanger 16 are isolated from one another so that thetwo air streams do not intermingle, thus helping to protect againstcontamination of the incoming outside air.

[0032] The outside air flowing through the duct 24 flows through otherducts (not shown) and usually is delivered to one of three locations.Either it is mixed with return air entering the system at 30 anddelivered to a heating/refrigeration unit 32 to either heat or cool theair, or it is delivered directly into the enclosed space 12, asindicated by the arrow 49.

[0033] It should be understood that in ventilating some spaces, such asin hospitals and other critical environments, it is desired to use 100%outside air for heating or cooling in the enclosed space, thusmaximizing the protection against air contamination. In suchcircumstances, the outside air is not mixed with the return air.

[0034] Temperature sensors are provided at 84 and 86, the inlet to theexhaust duct 22 and the inlet to the outside air duct 18, respectively,or at other suitable locations. The temperature sensor 84 senses theactual indoor temperature, and provides a corresponding electricalsignal. Similarly, the temperature sensor 86 senses the outside airtemperature and provides a corresponding signal.

[0035] These signals are delivered to the microprocessor-basedcontroller shown in FIG. 4, which uses the temperature signals tocontrol the operation and to selectively control the exchange of heatbetween the exhaust and outside air streams and other functions to bedescribed below.

[0036]FIG. 5 is a perspective view of a typical installation of theventilating system 10 and a heating/refrigeration unit 32 on the roof 47of a building. The unit 10 is shown separated from the unit 32 for thesake of clarity in the drawings.

[0037] Return air from the building returns to the unit 32 through theduct 43 and part or all of it flows through the exhaust conduit 22,through the, eat exchanger (not shown in FIG. 5) and out through theexhaust outlet.

[0038] Outside air enters the unit 10 and flows either through the heatexchanger along path 46, or through a by-pass 66 and gravity louvers 76(to be described below) to the outside air duct 24, which is connectedto the unit 32 through a fitting 45.

HEATING MODE

[0039] The first mode of operation to be described is one in whichheating of the enclosed space is required by a furnace or other heatingmeans. Usually, the outside air temperatures are below 50° F. whenheating of the interior space in most commercial buildings is required.However, the outside air temperature at which heating is first neededcan be considerably lower, e.g., 35° F., for buildings with largeinternal heat sources, such as large banks of computers or intenselighting, or solar energy-absorbing surfaces, etc. Of course, thistemperature also can be higher, where the internal heat generation is ata low level.

[0040] In residential buildings, the temperature at which heating isfirst required usually is considerably higher, say 60° F. or 65° F.,because internal heat generation usually is lower than in mostcommercial buildings.

[0041] As it will be described below in connection with FIG. 4, theoperation of the ventilating system is controlled by a programmedmicroprocessor 96. The heating mode is started when the outside airtemperature reaches a pre-determined level which can be different foreach building or space within a building. For this reason, themicroprocessor is of the field-programmable variety so that the heatingmode start point can be set independently for each building.

[0042] The fans 26 and 28 are variable-speed fans. During the heatingmode of operation, preferably the fans 26 and 28 operate at or nearmaximum speed, thus providing pre-heating of the outside air enteringthe enclosed space so as to reduce the cost or heating the space.

[0043] Of course, the speeds of the fans 26 and 28 can be varied asdesired to increase or decrease the corresponding air flows as requiredby the enclosed space.

SUPPLEMENTAL COOLING MODE

[0044] In accordance with the present invention, a supplemental coolingmode of operation of the ventilating system is provided in which outsideair is taken in but heat transfer between the exhaust air and theoutside air is reduced or eliminated so that the cool air will not beheated substantially by the exhaust air and will be supplied at or nearthe outside temperature for use in cooling the interior of the enclosedspace.

[0045] Usually, this mode of operation occurs when the temperature inthe enclosed space, as sensed by the temperature sensor 84, is above adesired level, e.g., 70° F., so that cooling is needed, while theoutside air temperature is below the desired level. Typically, thesupplemental cooling mode will most often be desired when the outsideair temperature is in the 50° F. to 70° F. range.

[0046] By this means, the natural cooling potential of the outside aircan be used to cool the enclosed space, either with or without the useof refrigeration or other cooling.

[0047] This mode of operation is particularly advantageous because manycommercial buildings require cooling when outside temperatures arerelatively low. For example, because of the use of substantial amountsof heat-generating computers or other office machines, indoor lighting,groups of people, solar heating through building windows, etc., manycommercial buildings have a heat build-up which is not dissipated byonly moderately cool outside temperatures, so that cooling is required.

[0048] During this mode of operation, heat exchange between the exhaustair and outside air is reduced or eliminated by one of several differentmethods. Only the simplest one will be described here. That is to simplyslow or stop the exhaust fan 28 so that either less or no exhaust airwill pass through the heat exchanger and heat the incoming outside air.Other methods will be described below.

COOLING MODE

[0049] This mode of operation is used when the outside air temperatureis above that desired for the enclosed space. When the weather outsideis hot, and the air exhausted from the enclosed space is cooler thanthat coming in from outside, the exhaust air cools the outside air inthe heat exchanger 16 before it enters the enclosed space. If necessary,the outside air then can be sent to a refrigeration unit 32 to be cooledfurther. This can significantly reduce the load on the refrigerationunit and reduce the cost of hot weather air conditioning. Somede-humidification of the outside air also is accomplished.

[0050] As during the heating mode, preferably, both fans 26 and 28 areoperated at maximum speed.

OUTSIDE AIR BY-PASS

[0051] In accordance with another aspect of the present invention, aby-pass conduit 66 is provided for introducing outside air into theenclosed space 12 under selected conditions. By-pass conduit 66 also hasits own independently operable variable speed fan 68 for deliveringoutside air into the outside air duct 24.

[0052] One of the conditions in which the by-pass duct is useful isduring the supplemental cooling mode of operation described above.During this mode, heat transfer between the exhaust air and the outsideair can be prevented without stopping the exhaust fan 28 simply byslowing or stopping the fan 26 and running the fan 68. This allows warminterior air to be exhausted and cool outside air to be introduced, withless or no heating of the outside air.

[0053] Another advantage of the invention is that the by-pass can beused for two different functions; it can be used as a supplementalcooling mode by-pass, as described above, and in defrosting the heatexchanger.

LOUVER SYSTEM

[0054] The optional louver system for use in conjunction with theby-pass, and also for other functions, now will be described.

[0055] A set of gravity-type louvers is provided at 76 at the exit ofthe by-pass 66 into the duct 24.

[0056] A set of motorized louvers 70 is provided in a septum or wall 36separating the ducts 18 and 20.

[0057] Two other motorized louvers 72 and 74 are provided, respectively,at the entrances to the ducts 18 and 20.

[0058] Additional gravity-operated louvers 78 are provided at the outletfrom the housing 14 into the duct 24.

[0059] Gravity-operated louvers open in response to an air pressuredifferential in one direction, but close either under gravity orpressure in the opposite direction. In general, it is preferred to usegravity-type louvers instead of motor-driven louvers, wherever possiblebecause they tend to operate smoothly over long periods of time withoutsignificant maintenance and also are less complicated and expensive.Gravity louvers work either in a vertical or horizontal orientation.

[0060] The operation of the louvers during various different phases ofthe operation of the system will be described below.

DEFROST MODE

[0061] The exhaust air from the enclosed space during the cooling modehas a certain level of humidity, say, 55% or thereabouts. When the warm,humid air from the interior of the enclosed space passes through thecold heat exchanger, moisture condenses from the air and condensate andwater runs downwardly out of the heat exchanger and through holes inperforated floor panels 54 and 56 of the housing 14 to drip pans 58 and60 with drains 62 and 64 for removing the accumulated water. The drippans are isolated from one another by a septum or wall 59 in order topreserve the isolation of the outside air path from the exhaust airpath.

[0062] When the outside air temperature is very low, the wateraccumulating in the heat exchanger starts to freeze and clog up thepassages in the heat exchanger. This reduces the heat exchangeefficiency of the heat exchanger, increases the pressure drop across theheat exchanger, and can totally disable it. Therefore, means areprovided for defrosting the heat exchanger when freezing conditions aredetected.

[0063] Freezing conditions are detected preferably by means of atemperature sensor 88 mounted near the lower portion of the heatexchanger where ice tends to accumulate first. When the temperaturesensed by the sensor 88 reaches freezing (32° approximately), thetemperature sensor sends a signal to the control system which starts thedefrost operation.

[0064] In its simplest form, the defrost operation comprises simplyreducing the speed of the outside air fan 26 while leaving the speed ofthe exhaust fan 28 at its original maximum speed, thus reducing thecooling of the heat exchanger and allowing the warmth of the exhaust airto melt the ice in the heat exchanger and bring its temperature up toabove the freezing level. When the temperature sensed by the sensor 88rises to the desired level again, the speed of the fan 26 is restored toits previous level.

[0065] This operation is repeated as often as necessary to prevent icingof the heat exchanger.

[0066] The simple defrosting method described above is adequate in manycircumstances. However, more heating of the heat exchanger may berequired in order to defrost it. If so, the intake of outside air can bestopped completely for a time until the temperature of the heatexchanger rises.

[0067] In accordance with another aspect of the invention, if it isdesired to maintain the flow of outside air into the enclosed space at asteady level, even during defrost, then the fan 68 in the by-pass duct66 can be turned on to bring in outside air without passing it throughthe heat exchanger, to either supplement the air brought in by theslowed fan 26, or to replace it entirely.

[0068] It is possible that further heating of the heat exchanger beyondthat provided by the means described so far would be necessary. In suchcases, by closing the louvers 72 and 74 and opening the louvers 70, theexhaust air is re-circulated back through the outside air flow passagesand into the enclosed space, thus using the residual heat in the exhaustair for further heating and defrosting. Thus, exhaust air exiting theheat exchanger can pass upwardly from conduit 20 into conduit 18,through the outside air passages in the heat exchanger, and out throughthe duct 24 back into the enclosed space.

[0069] Even further heating of the heat exchanger can be provided byother means such as the introduction of steam into the inlet 91 in theduct 18 so as to preheat the outside air before it reaches the heatexchanger. Of course, this requires additional energy and should berestricted to uses in which it is considered most beneficial, such as inhospitals and other institutions. Other heat sources also can be used tosupply the necessary supplemental heat.

[0070] When the by-pass fan 68 is operated, the air pressure it produceslifts the louvers 76 and allows air to pass into the duct 24. If thereis no air flow created by the fan 26 through the louvers 78, the backpressure produced by the fan 68 closes the louvers so that outside airdoes not flow backwardly through the heat exchanger.

[0071] Means other than a temperature sensor can be used to detectfreezing conditions. For example, air pressure sensors to detect thechange in pressure caused by ice formation are known in the prior artcan be used, if desired.

EVAPORATIVE COOLING

[0072] During the cooling mode of operation, it is preferred to use arelatively low-cost method of further reducing the temperature ofincoming outside air so as to decrease the cooling load on therefrigeration system. This is provided by an evaporative cooling systemincluding a spray nozzle 94 (FIG. 1) and a solenoid-operated valve 92selectively supplying pressurized water from the supply line 90 to thespray nozzle 94. The spray 94 sprays water into the exhaust air beforeit enters the heat exchanger 16.

[0073] Preferably, the water from the spray nozzle 94 is sprayed onto anair-permeable membrane 82 which covers the exhaust air entrance to theheat exchanger.

[0074]FIG. 2 is a perspective view of the heat exchanger 16 showing themembrane 82 (broken away). The membrane 82 preferably comprises a thinmat of synthetic fibers such as those used in ordinary air filters so asto enhance the evaporation of the water in the exhaust air stream togive evaporative cooling of the exhaust air. Such a mat is made offibers which do not deteriorate due to prolonged contact with water andthe air which impinges on the membrane.

[0075] Alternatively, as shown in FIG. 2, water can be dripped from oneor more pipes 83 with holes 85 in it to drip water onto the membrane.The water migrates downwardly through the membrane under the force ofgravity.

[0076] Any water which accumulates in the heat exchanger due to thewater spray will drain out through the bottom of the heat exchanger andinto the drip pans 58 and 60, the same as condensate.

[0077] Evaporative cooling can reduce the temperature of the incomingair by a very significant amount, and is not very costly in terms ofeither materials or energy required. Therefore, it is a verycost-effective way of preconditioning the outside air to reduce theenergy requirements of the refrigeration system. Again, as with otheroperations of the system, the evaporative cooling equipment preferablyis turned on in response to the detection of an outside air temperaturewhich is greater than the desired inside air temperature by a certainminimum amount.

[0078] For example, the minimum temperature difference in question mightbe 3 to 10 degrees Farenheidt. Thus, if the outside temperature were 72degrees and the desired space temperature is 70° F. and the minimumdifferential is 10 degrees, the evaporative cooling system would notoperate. When the outside air temperature reaches 80 degrees, theevaporative cooling system will turn on and operate continuously untilthe outside air temperature drops below the desired level.

CONTROL SYSTEM

[0079]FIG. 4 shows schematically the control circuit of the ventilationsystem of the present invention. A microprocessor 96 is provided andprogrammed so as to control both the turning on and off and the speed ofeach of the fans 26, 28 and 68 in response to the signals sent to themicroprocessor by the temperature sensors 84, 86 and 88. Operatingsignals are sent by the microprocessor also to the louver motors 71, 73and 75 to operate the powered louvers and the solenoid valve 92 to startand stop the water spray for the evaporative cooling system describedabove.

[0080] As noted above, it is preferred that the microprocessor befield-programmable to allow the variation of set-points, etc. for eachinstallation.

[0081] The microprocessor also is programmed to have certain“dead-bands” around the various control points to prevent excessive“hunting”. Preferably, the dead-bands also are field-programmable inorder to enable the customization of the system for a particularenclosed space.

[0082] For example, a dead-band of 3° F. to 5° F. or more around eachset-point can be beneficial. Manual over-ride also can be provided toenable adjustments for special circumstances. Automatic control of someset-points also can be provided. For example, the switch-over fromsupplemental cooling mode to heating mode can be delayed, even though asudden cold-snap reduces the outside air temperature to below theheating mode set-point, if the inside air temperature is still highenough to require cooling.

HEAT EXCHANGER

[0083] The heat exchanger 16 has a rectangular shape and preferably ismade of plastic. It is preferred that the heat exchanger be of the typeshown in U.S. Pat. No. 4,820,468 to M. J. Hartig, which is sold by theHartig Company, Wilmington, Del.

[0084] The structure of this heat exchanger is illustrated in FIGS. 2and 3, and particularly in FIG. 3. The heat exchanger structurecomprises a plurality of plastic extrusions 100 with closely spacedparallel passageways 104 separated by square extruded channel members102 extending perpendicular to the direction of the passageways 104.Although only two of the extrusions 100 and a pair of channel membersare shown in FIG. 3, for the sake of simplicity in the drawings, itshould be understood that there are many extrusions and channel membersin the typical heat-exchanger.

[0085] Each extrusion 100 comprises a solid top sheet 101 and a solidbottom sheet 103 with multiple vertical walls forming the passageways104. Thus, crossed air flow paths are formed by the passageways 104, onthe one hand, and the spaces 106 between the channel members and thehollow interiors of the members 102. These crossed flow paths areisolated from one another by the solid sheets 101 and 103. Theextrusions 100 and 102 are heat-welded together to form a strong,lightweight corrosion-resistant heat exchanger.

[0086] The exhaust air preferably flows through the larger passageways106, as indicated by the arrow 50, and the outside air flows through thepassageways 104. This arrangement is preferred because the exhaust airmay have entrained water droplets and condensation and ice may form inthe exhaust air passageways so that the larger passageways will remainoperative for heat transfer over a wider range of operatingcircumstances than if the smaller passages were used. Althoughcondensation also will occur when hot, humid outside air is cooled inthe heat exchanger, it is believed that the larger passageways willbetter suit the conduct of exhaust air.

[0087] The material of which the heat exchanger 16 is made preferably ispolyethylene or polypropylene, or other plastic materials which also areimpervious to deterioration under prolonged contact with water andflowing air.

[0088] Equivalent heat exchangers also can be used in the practice ofthe invention. For example, isolating heat exchangers made of variousmetals can be used, as well as heat pipes whose ends are isolated fromone another with one end in the outside air flow and the other in theexhaust air flow. Hydronic heat exchangers with liquid working fluidsalso can be used.

[0089] The plastic heat exchanger described above is advantageous overthe usual metal heat exchanger, even though the heat conductivity of theplastic is considerably lower than that of the metal. The plastic lastsa very long time without corroding and is considerably less expensivethan metal. Also, the plastic heat exchanger is less expensive tomanufacture than metal heat exchangers. The added volume required forthe plastic heat exchanger to exchange the same amount of heat as ametal heat exchanger is more than offset by the foregoing advantages.

[0090] The plastic heat exchanger is believed to be particularlyadvantageous when used with evaporative cooling because any scale whichforms from the water spray can be broken free relatively easily byflexing the heat exchanger.

[0091] The above description of the invention is intended to beillustrative and not limiting. Various changes or modifications in theembodiments described may occur to those skilled in the art. These canbe made without departing from the spirit or scope of the invention.

What is claimed is:
 1. A method of ventilating an enclosed space usingan air handling system including an isolating heat exchanger forconducting exhaust air from said enclosed space and outside air intosaid enclosed space and exchanging heat between said exhaust air andsaid outside air while isolating the flows of said outside and exhaustair from one another, an enclosed space air temperature sensor, anoutside air temperature sensor, a first fan to move said exhaust airthrough said heat exchanger, a second fan to move said outside airthrough said heat exchanger, and a programmed controller for controllingthe flow of said exhaust air and said outside air and the heat transfertherebetween, said method comprising: (a) during a first cooling mode,causing heat to be transferred from said outside air to said exhaust airwhen the outside air temperature is greater than a desired enclosedspace temperature; (b) during a second cooling mode, reducing oreliminating the transfer of heat from said exhaust air to said outsideair through said heat exchanger when said outside air temperature islower than said desired enclosed space temperature, and said spacetemperature is greater than said desired space temperature.
 2. A methodas in claim 1 including the step of, during a first heating mode,utilizing said controller in causing heat to be transferred from saidexhaust air to said outside air when said outside air temperature is ata temperature which is below said desired space temperature by at leasta pre-determined amount.
 3. A method as in claim 2 in which said airhandling system includes a sensor for detecting freezing conditions insaid heat exchanger, and including the step of utilizing said controllerin causing the heating of said heat exchanger to alleviate said freezingcondition.
 4. A method as in claim 3 in which the heat exchanger heatingstep includes reducing the flow of sub-freezing outside air through saidheat exchanger.
 5. A method as in claim 3 in which the heat exchangerheating step includes re-circulating said exhaust air through said heatexchanger in the path otherwise taken by said outside air.
 6. A methodas in claim 3 in which the heat exchanger heating step includes one ormore steps selected from the group consisting of; reducing the flow ofoutside air through said heat exchanger while maintaining the flow ofexhaust air therethrough; recirculating said exhaust air through saidheat exchanger; and applying heat from a heat source to said heatexchanger.
 7. A method as in claim 1 including the step of usingevaporative cooling during said first cooling mode to cool said exhaustair prior to its entering said heat exchanger.
 8. A method as in claim 7in which said evaporative cooling comprises one or more steps selectedfrom the group consisting of; spraying water into said exhaust air;wetting a membrane with water and contacting said exhaust air with saidmembrane; and spraying water into said exhaust air and onto a membranecontacting said exhaust air.
 9. A method as in claim 1 in which said airhandling system includes a by-pass duct and fan for selectivelyintroducing outside air into said enclosed space without flowing throughsaid heat exchanger; and including the step of operating the by-pass fanto introduce outside air into said space during said second coolingmode.
 10. A method as in claim 9 including the step of operating saidby-pass fan during defrosting of said heat exchanger.
 11. A method as inclaim 1 in which the step of substantially eliminating the transfer ofheat during said second cooling mode is performed by reducing oreliminating the flow of exhaust air through said heat exchanger whileflowing said outside air therethrough.
 12. A method of ventilating anenclosed space using an air handling system including an isolating heatexchanger for conducting exhaust air from said enclosed space andoutside air into said enclosed space and exchanging heat between saidexhaust air and said outside air while isolating the flows of saidoutside and exhaust air from one another, an enclosed space airtemperature sensor, an outside air temperature sensor, a first fan tomove said exhaust air through said heat exchanger, a second fan to movesaid outside air through said heat exchanger, and a programmedcontroller for controlling the flow of said exhaust air and said outsideair and the heat transfer therebetween, said method comprising: during aheating mode, causing heat to be transferred from said exhaust air tosaid outside air when said outside air temperature is at a firsttemperature which is below a desired space temperature by at least apre-determined amount, and during a cooling mode, reducing oreliminating the transfer of heat from said exhaust air to said outsideair through said heat exchanger when said outside air temperature isabove said first temperature but below said desired temperature, andsaid space temperature is greater than said desired space temperature.13. A method of ventilating an enclosed space using an air handlingsystem including an isolating heat exchanger for conducting exhaust airfrom said enclosed space and outside air into said enclosed space andexchanging heat between said exhaust air and said outside air whileisolating the flows of said outside and exhaust air from one another, anenclosed space air temperature sensor, an outside air temperaturesensor, a first fan to move said exhaust air through said heatexchanger, a second fan to move said outside air through said heatexchanger, and a programmed controller for controlling the flow of saidexhaust air and said outside air and the heat transfer therebetween,said method comprising: cooling said enclosed space by substantiallyeliminating the transfer of heat from said exhaust air to said outsideair through said heat exchanger when said outside air temperature isbelow a desired space temperature but above a temperature at whichheating of the enclosed space is required, and said space temperature isgreater than said desired space temperature.
 14. A method as in claim 13in which the transfer of heat from said exhaust air is reduced oreliminated by one of more of the steps consisting of: by-passing outsideair around said heat-exchanger; and reducing or eliminating the flow ofexhaust air through said heat exchanger
 15. A system for ventilating anenclosed space, said system including an outside air duct, an inside airduct, an inside air temperature sensor, an outside temperature sensor, aheat exchanger with two sets of isolated air flow passages, one of whichcommunicates with said outside air duct, and the other of whichcommunicates with said inside air duct, an inside air fan positioned toforce inside air through said inside air duct and said heat exchanger,an outside air fan positioned to force said outside air through saidoutside air duct and said heat exchanger, and a control circuitincluding a programmed processor for selectively operating said air fansand the exchange of heat between said inside air and said outside air inresponse to temperature signals received from said temperature sensors.16. A system as in claim 15 including an evaporative cooler forselectively cooling said inside air before it passes through said heatexchanger.
 17. A system as in claim 15 in which said evaporative coolerincludes a membrane in the flow path of said inside air and a watersprayer to supply water to wet said membrane.
 18. A system as in claim15 including a sensor for sensing freezing conditions in said heatexchanger, said processor being programmed to cause said heat exchangerto be heated when the freezing conditions are detected.
 19. A system asin claim 18 in which said processor is programmed to cause said heatexchanger to be heated by reducing the flow of outside air through saidheat exchanger while maintaining the flow of inside air therethrough.20. A system as in claim 15 including a by-pass conduit and a fan forintroducing outside air into said enclosed space without passing saidoutside air through said heat exchanger, said processor being programmedto cause outside air to flow through said by-pass in response to signalsindicating that the outside air temperature is below a desired airtemperature in said space and above a temperature at which heating insaid space is needed.
 21. A system as in claim 15 in which saidprocessor is programmed to cause heat to be transferred from said insideair to said outside air in said heat exchanger when said inside airtemperature sensor indicates that heating is needed in said enclosedspace, to cause heat to be transferred from said inside air to saidoutside air when said inside air temperature sensor indicates thatcooling is needed in said enclosed space and said outside airtemperature sensor indicates that said outside air temperature is higherthan the inside air temperature, and to cause no heat to be transferredbetween said inside air and outside air when cooling in said space isneeded and said outside air temperature is below said inside airtemperature.
 22. A system as in claim 15 in which said heat exchanger ismade of crossed plastic conduits assembled together.
 23. A system as inclaim 20 in which said by-pass conduit has an outlet into said outsideair duct, and including gravity louvers normally covering said outlet ofsaid by-pass conduit, said louvers being opened by air pressure fromsaid by-pass fan.
 24. A system as in claim 18 in which said inside andoutside ducts have inlet sections, and including louvers at the inletsections and another louver between said sections, said louvers beingselectively operable to recirculate inside air through said heatexchanger and reduce or eliminate outside air flow in response to thedetection of freezing conditions in aid heat exchanger.